As high-performance rare-earth magnets, two kinds of magnets, i.e., a samarium-cobalt-based magnet and a neodymium-iron-boron magnet, are widely used.
Especially, since the neodymium-iron-boron magnet (“R-T-B-based magnet”, hereinafter) shows the highest magnetic energy product among various magnets and the price thereof is relatively low, this magnet is employed for various electric devices.
The rare-earth magnet is produced in the following manner: a raw-material alloy is coarsely pulverized and finely pulverized to form alloy powder, the alloy powder is formed by pressing, and it is subjected to a sintering step and a thermal processing step. When the rare-earth magnet is produced, during the process for coarsely pulverizing the raw-material alloy, since the pulverizing efficiency is high, hydrogen pulverizing operation is frequently used.
The hydrogen pulverizing operation is a technique in which hydrogen is stored in a raw-material alloy to make it brittle, thereby pulverizing the raw-material alloy. This operation is carried out by doing the steps.
First, an alloy which is a raw material is inserted into a hydrogen furnace and then, an interior of the hydrogen furnace is decompressed by evacuation (vacuuming). Thereafter, hydrogen gas is supplied into the hydrogen furnace and the raw-material alloy is made to store hydrogen (hydrogen storing step). After a predetermined amount of time has elapsed, the raw-material alloy is heated (heating step) while evacuating the interior of the hydrogen furnace, and hydrogen is discharged from the raw-material alloy. Thereafter, the raw-material alloy is cooled (cooling step) and the hydrogen pulverizing operation is completed. Accordingly, the raw-material alloy is made brittle and coarsely pulverized powder is obtained.
The coarsely pulverized powder after the hydrogen pulverizing operation is pulverized into fine pulverized powder of a few μm.
A rare-earth element itself is an active element, and if the rare-earth element touches atmosphere, it is oxidized. Therefore, in the case of a magnet using the rare-earth element, an antioxidant operation in respective producing steps is effective for enhancing magnetic properties. Thus, antioxidant measures are employed in the steps.
For example, there is a technique (patent document 1) in which fine pulverized powder after a fine pulverizing operation is put directly into mineral oil and then, the pulverized powder is formed, thereby lowering oxygen in sintered compact. Also, there is a technique (patent document 2) in which liquid lubricant is added to fine pulverized powder after a fine pulverizing operation such that surfaces of the particles are covered, thereby preventing oxidation of the fine pulverized powder. These methods propose to lower the oxygen content of fine pulverized powder.
In a step of producing a rare-earth magnet, even coarsely pulverized powder of a raw-material alloy for rare-earth magnets having relatively large particles, it is known that if the coarsely pulverized powder touches atmosphere during steps, oxidation abruptly proceeds and oxygen content is increased and magnetic properties of a finally obtained sintered magnet are deteriorated.
As a method of obtaining a raw-material alloy before a rare-earth magnet is coarsely pulverized, a strip casting method, which is one of quenching methods, is currently used frequently because a sintered magnet having high magnetic properties can be obtained in the end. As another quenching method, a centrifugal casting method is proposed.
A thickness of a raw-material alloy for rare-earth magnets produced by the quenching method is in a range of 0.03 mm or more and 10 mm or less. A thickness of the raw-material alloy for rare-earth magnets produced by the strip casting method is 1 mm or less.
Since a raw-material alloy produced by the quenching method is cooled within a relatively short amount of time as compared with a raw-material alloy produced by a conventional ingot casting method (mold casting method), the structure is miniaturized and a crystal grain size is small. A total area of a grain boundary is large and dispersibility of R-rich phase is also excellent.
Further, since a raw-material alloy produced by the quenching method is easily fractured at a grain boundary by a hydrogen pulverizing method, the R-rich phase easily appears on the particle surface of the obtained alloy powder. Since R of the R-rich phase easily reacts with oxygen, powder of a raw-material alloy produced by the quenching method oxidizes extremely easily and its magnetic properties are deteriorated sorely.
To prevent oxidization of coarsely pulverized powder (hydrogen pulverized powder) after hydrogen pulverizing operation, there is proposed a technique (patent document 3) in which hydrogen pulverized powder is sent to a fine pulverizing step in inert atmosphere. Additionally, another technique (patent document 4) is also proposed in which a step in a recovery chamber for discharging hydrogen pulverized powder from a hydrogen pulverizer is carried out in inert gas.